1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet and an image forming apparatus for recording image information on the fed sheet and, more specifically, to a sheet conveying apparatus which adapts a sheet feeding pressure to a sheet size or a loaded weight.
2. Description of the Related Art
In the fields of image forming apparatuses such as a copying machine, a facsimile machine, and a printer, a sheet feeding apparatus has a sheet storing device such as a sheet tray on which a sheet is loaded and which stores a sheet. A sheet loading plate on which a sheet stack is loaded and supported is arranged in the housing of the sheet storing device. A pickup roller (sheet feeding roller) is arranged above the sheet loading plate such that the pickup roller is brought into contact with the uppermost sheet. The sheet loading plate is biased by a biasing spring lifts up the sheet stack to press the uppermost sheet against the pickup roller. In this case, a force of pressing the sheet stack against the pickup roller is called a sheet feeding pressure hereinafter. When a rotating power is transmitted to the pickup roller, a rotation frictional force of the roller and a bias spring force from the lower side generate a sheet feeding pressure. In this manner, the uppermost sheets are sequentially fed and conveyed to the image formation device.
As a known means which generates a sheet feeding pressure, in addition to a biasing spring, a structure in which a lifting mechanism constituted by a drive motor and the like is moves upwardly to lift a sheet loading plate to a position at a level of a conveyance path level to bring a pickup roller into press contact with the uppermost sheet by a spring is known.
Since the structure which obtains the sheet feeding pressure by a biasing spring is simple and inexpensive, the structure is often used in the field of a low-speed-processing image forming apparatus having a sheet tray and a sheet cassette in which a capacity of loaded sheets is 25 or less. On the other hand, in the latter structure in which a sheet feeding pressure is obtained by a lifting mechanism, a constant sheet feeding pressure can be always obtained regardless of a sheet live load, and a large number of sheets can be loaded and stored. However, the number of parts increases, and a control mechanism which lifts the sheet loading plate up is required to increase the cost. For this reason, the structure is used in the field of an image forming apparatus which performs relatively high speed processing.
The sheet feeding pressure can be expressed by a value obtained by subtracting an additional value between a sheet loading weight and the weight of the sheet loading plate from a push-up force acting on the sheet loading plate. When the push-up force obtained by the biasing spring is excessively large more than necessary, so-called multi feeding easily occurs such that several sheets are conveyed by a pickup roller at once. On the other hand, when a push-up force generated by a biasing spring is small not to obtain a necessary sheet feeding pressure, the pickup roller slips on the uppermost sheet and cannot feed out the sheet to easily cause non-feeding.
In a sheet cassette which copes with a relatively small capacity of 250 loaded sheets or less, a change in weight caused by the number of sheets is small. For this reason, it is relatively easy to appropriately adjust or set a sheet feeding pressure. However, in a large-capacity sheet cassette on which 500 sheets can be loaded, the following problem occurs. A change in weight caused by the number of loaded sheets is large. Furthermore, when a so-called universal cassette which can cope with several types of sheets having different sizes is used, a difference in weight between a sheet of the maximum size and a sheet of the minimum size is so large that appropriate setting of a sheet feeding pressure is very difficult.
A sheet feeding pressure generated by using a biasing spring, as shown in a characteristic graph in FIG. 7, exhibits an almost linear characteristic from a full load condition to a feeding condition of a last sheet regardless of sheet sizes. In this case, for example, setting is performed in accordance with a sheet feeding pressure obtained in a full load condition of sheets having a maximum size, i.e., A3, a sheet feeding pressure coping with a full load condition of sheets of minimum sizes, i.e., B5R is excessively large more than necessary. As a result, when a sheet of a size B5R is sent, multi feeding easily occurs.
In contrast to this, as shown in a characteristic graph in FIG. 8, when a sheet feeding pressure is set in accordance with a full load condition of size B5R sheets, a sheet feeding pressure in a full load condition of size A3 sheets, a sheet feeding pressure in the full load condition of size A3 sheets is short to make it impossible to feed the sheets. More specifically, it is impossible to obtain a sheet feeding pressure adapting to a full load condition of sheets having different sheet sizes to a last-sheet-feeding condition by a single biasing spring having a constant spring strength (spring constant).
In order to solve the problem, for example, in Japanese Patent Application Laid-open No. 9-30663, the following configuration is proposed. For example, in Japanese Patent Application Laid-open No. 9-30663, several types of biasing springs having different spring strengths are prepared, and the spring strengths are changed in accordance with at least one biasing spring for each of the different types of sheets. Japanese Patent Application Laid-open No. 61-22647 proposes the following configuration. An inclined guide unit is arranged on the lower surface of a sheet loading plate to displace a contact portion of a spring depending on a sheet width, so that a sheet feeding pressure is kept almost constant.
However, as in an invention described in Japanese Patent Application Laid-open No. 9-30663, when several types of springs are prepared for respective sheets of different sizes, another dedicated member such as a pressure-adjusting arm must be arranged for each spring to change a spring strength depending on a sheet size. For this reason, in addition to an increase in number of parts or an increase in cost caused by preparing several types of springs, an apparatus is complicated due to the pressure-adjusting arm or the like, and management of accuracy of spring strengths becomes very difficult.
As described in Japanese Patent Application Publication No. 61-22647, when the inclination guide device is arranged on the lower surface of the sheet loading plate to change the contact portion of the biasing spring depending on sheet widths, a space for the inclination must be counted in a direction of height of the sheet cassette. As a result, the sheet cassette increases in size to increase in size of the apparatus. Since a compression coil spring is used as a biasing spring, a guide member for guiding an extension/contraction operation must be independently arranged. At the same time, a parts cost and a manufacturing cost are disadvantageously increased by an increase in number of parts.